The present invention relates to a low-temperature sintered porcelain composition which is suitable for forming (particularly, injection molding), and which enables the resin removal process after forming of the primary molded product to be carried out under favorable condition.
Generally, well known plastics such as phenol resin, polyester resin and urea resin are superior in various characteristics including formability, chemical resistance, electrical characteristics, etc.
However, these plastics also have such shortcomings as being easy to burn and producing a large amount of toxic gas or smoke when burnt.
On the other hand, conventional known ceramics and vitreous compositions, which are fabricated through sintering, melting and forming for a longer period of time under a high temperature of more than 1000.degree. C., have such shortcomings as requiring high-temperature treatment and hence consuming a large amount of fuel.
Moreover, low-temperature sintered porcelains and low-melting point inorganic compositions have also been researched and developed in recent years.
These products are, however, inevitably subjected to the process of heating and forming, for example, utilizing the calcined materials or sintering after reaction by heating, and have not so far come into wide use because of the intricate process required.
In view of the above, the applicant of this patent application did research on a low-temperature sintered porcelain which uses a composition consisting of a vitreous component, aluminum oxide, clay component, binder and a coloring agent, and the manufacturing method thereof. Two patent applications were filed as a result of such research (see Japanese Patent Public Disclosure Nos. 54-53118 and 57-67075).
But, it was practically impossible or very difficult for the above low-temperature sintered porcelain composition to be directly subjected to injection molding or the like due to its specific plasticity and fluidity.
For this reason, there has been proposed still another method as follows. Organic material including a thermoplastic resin such as polystyrene or atactic polypropylene (APP), lubricant, plasticizer, etc., which are usually employed in injection molding of ceramics, are mixed and kneaded with powder of the above low-temperature sintered porcelain composition to make a pellet-like or powdered forming material. This material is then injected into a mold of the forming machine, in a similar way to injection molding of plastics, thereby to fabricate a molded product (primary molded product). Finally, the foregoing organic materials are decomposed and volatilized in a resin removing furnace to carry out the main sintering, so that there can be obtained a complete molded product. (See Japanese Patent Publication Nos. 36-7883 and 51-29170)
However, the above method has another shortcoming. Namely, polystyrene contained in the powdered forming material is a resin suitable for imparting the proper formability, but it causes pyrolysis at a brush in the resin removing process under high temperature, e.g., about 290.degree. C. to 330.degree. C. As a result, there will be caused swells, cracks, etc. after the main sintering, and this makes it impossible to obtain a good final product.